Lithography methods are widely used in the production of microscopic structures in a variety of electronic devices such as semiconductor devices and liquid crystal devices, and ongoing miniaturization of the structures of these devices has lead to demands for further miniaturization of the resist patterns used in these lithography processes. With current lithography methods, using the most up-to-date ArF excimer lasers, fine resist patterns with a line width of approximately 90 nm are able to be formed, but in the future, even finer pattern formation will be required.
In order to enable the formation of these types of ultra fine patterns of less than 90 nm, the development of appropriate exposure apparatus and corresponding resists is the first requirement.
With respect to resists, chemically amplified resists, which enable high levels of resolution to be achieved, are able to utilize a catalytic reaction or chain reaction of an acid generated by irradiation, exhibit a quantum yield of 1 or greater, and are capable of achieving high sensitivity, are attracting considerable attention, and development of these resists is flourishing.
In positive chemically amplified resists, resins having acid dissociable, dissolution inhibiting groups are the most commonly used. Examples of known acid dissociable, dissolution inhibiting groups include acetal groups such as ethoxyethyl groups, tertiary alkyl groups such as tert-butyl groups, as well as tert-butoxycarbonyl groups and tert-butoxycarbonylmethyl groups. Furthermore, structural units derived from tertiary ester compounds of (meth)acrylic acid, such as 2-alkyl-2-adamantyl (meth)acrylates, are widely used as the structural units containing an acid dissociable, dissolution inhibiting group within the resin component of conventional ArF resist compositions, as disclosed in Patent Document 1 listed below.
On the other hand, with respect to the exposure apparatus, techniques such as shortening the wavelength of the light source used, and increasing the diameter of the lens aperture (NA) (namely, increasing NA) are common. For example, for a resist resolution of approximately 0.5 μm, a mercury lamp for which the main spectrum is the 436 nm g-line is used, for a resolution of approximately 0.5 to 0.30 μm, a similar mercury lamp for which the main spectrum is the 365 nm i-line is used, for a resolution of approximately 0.3 to 0.15 μm, 248 nm KrF excimer laser light is used, and for resolutions of approximately 0.15 μm or less, 193 nm ArF excimer laser light is used. In order to achieve even greater miniaturization, the use of F2 excimer laser (157 nm), Ar2 excimer laser (126 nm), extreme ultraviolet radiation (EUV: 13 nm), electron beam (EB), and X-ray and the like is also being studied.
However, shortening the wavelength of the light source requires a new and expensive exposure apparatus. Furthermore, if the NA value is increased, since the resolution and the depth of focus exist in a trade-off type relationship, even if the resolution is increased, a problem arises in that the depth of focus is lowered.
Against this background, a method known as immersion exposure (immersion lithography) has been reported (for example, see Non-Patent Documents 1 to 3). In this method, exposure (immersion exposure) is conducted in a state where the region between the lens and the resist layer formed on a wafer, which has conventionally been filled with air or an inert gas such as nitrogen, is filled with a solvent (a immersion medium) that has a larger refractive index than the refractive index of air.
According to this type of immersion exposure, it is claimed that higher resolutions equivalent to those obtained using a shorter wavelength light source or a larger NA lens can be obtained using the same exposure light source wavelength, with no lowering of the depth of focus. Furthermore, immersion exposure can be conducted using existing exposure apparatus. As a result, it is expected that immersion exposure will enable the formation of resist patterns of higher resolution and superior depth of focus at lower costs. Accordingly, in the production of semiconductor devices, which requires enormous capital investment, immersion exposure is attracting considerable attention as a method that offers significant potential to the semiconductor industry, both in terms of cost and in terms of lithography properties such as resolution. Currently, water is mainly used as the immersion medium for immersion lithography.